The new safety interlock device disclosed herein is for use in appliances such as household washing machines and dryers for maintaining the access door locked at any time that a motor driven component such as the spin dry basket of a washing machine is being driven or is coasting to a stop. For centrifugal drying the basket is rotated at high speed so there is chance for a person to be injured if the basket can be accessed while still rotating U.S. Pat. No. 4,286,811, describes a bimetal actuated locking device and switch which has some of the basic features of the improved switch design described herein. It is a unitary device including a switch and a pivotal switch operating arm mounted on a base. The switch operating arm is biased rotationally in one direction. When the lid of an appliance is closed, the operating arm is rotated and a circuit is completed through a bimetal element in a device and through the motor that drives the basket of the washing machine during the spin dry phase of the machine operating cycle. The motor is energized provided the timer switch customarily found in washing machines has otherwise closed the circuit through the motor to enable it to be run during the spin drying phase.
When the lid is closed, the bimetal heats and deflects and thereby causes a latch arm to swing into engagement or locking relation with a latch element. This locks the lid closed. The lid remains latched until the motor has been de-energized long enough for the bimetal element to cool and deflect oppositely. The resulting delay period precludes access to the basket or other moving component of the machine until the basket has coasted to a complete stop.
One of the problems with conventional bimetal-operated appliance locks is that when a motor is heavily loaded because of a large load in the basket of the washing machine during its drying cycle, heavy electric current flows through the bimetal and exceeds its rating. In other words, the bimetal deflects excessively which can result in permanent distortion and loss of calibration. It can also cause degradation of plastic parts in the switch and lock assembly.
One solution to the excessive heating and excessive deflection of the bimetal has been invented by Richard Case, assignor to White Consolidated Industries. His solution is to connect shunting contacts in parallel with the bimetal blade and cause the shunting contacts to close when the blade travel reaches a desired excursion. Closure of the shunt switch causes the bimetal to cool momentarily until the contacts open thereby limiting the bimetal temperature through contact cycling. A problem associated with this concept is that any restriction of the lock lever can prevent sufficient excursion of the bimetal to close the shunting contacts, thereby permitting the blade to overheat anyway.
The bimetal switch actuating member used in safety lock switches of the kind under discussion, even if they are from the same manufacturing batch, exhibit variances in straightness. In prior safety lock switches, no means have been provided for compensating the variances in straightness which means that there is no certainty that the bimetal member will be in the desired neutral position when it is cold. Combination lid lock and switching devices are installed in appliances such as washing machines where they are vulnerable to being splashed with water. In mechanical lock/switches, there must be a shaft extending from the inside to the outside of the switch housing to swing the locking lever. This requires some kind of seal about the shaft. Conventional seals have imposed a substantial frictional drag on the shaft which can propagate a restraining force on the bimetal. A seal that imposes little drag on the operating shaft would be desirable because the shaft is operated by a force derived from the bimetal and this is a low-level force. A correlative problem is to mount the shaft in bearings that will contribute toward achieving a good splashproof seal between the inside and outside of the switch housing and will impose minimal frictional drag on the operating shaft.